In thermal ink-jet printheads, thin film resistors are employed as heaters to form a bubble of ink over the resistor surface. The growth and collapse of the bubble causes an ink droplet to be ejected from an orifice associated with the resistor. The ejected droplet of ink is directed toward a medium, such as paper.
At a predetermined time, as determined by a signal sent to the printer from, say a computer, the resistor is heated (by I.sup.2 R heating) to a temperature sufficient to vaporize a thin layer of ink directly over the resistor, which rapidly expands into a bubble. This expansion, in turn, causes part of the ink remaining between the resistor and the orifice to be expelled through the orifice toward the medium. In present use, the resistor is heated to provide a surface temperature of a few hundred degrees, at repetition frequencies up to 50 kHz and above. However, heating of the resistor itself lasts less than about 10 .mu.sec.
The presence of wall-like structures, commonly called "barriers", in the immediate vicinity of a thermal ink-jet resistor has significant effects on the performance of the device.
When a vapor bubble collapses over a resistor which has no barrier structure in its immediate vicinity (barriers which are several mils away have little effect), the event approximately has axial symmetry with the final collapse point at the center of the resistor. In this case, fluid can flow freely from all directions as the bubble collapses.
When a wall or barrier is placed near the resistor, refill cannot occur from this direction, thus the bubble appears to be pushed towards the wall by fluid filling from all other directions. A single-sided barrier structure for an array of resistors is impractical to implement, since it would not actually isolate adjacent resistors, which is the original function of the barrier. A two-sided barrier configuration causes refill to occur from two directions; the final stages of bubble collapse occurs in an approximate line across the center of the resistor. Thus, the single collapse point (which in practice may be a small area) is spread into a line which reduces the rate or magnitude of impacting at any one point on the line. However, the bubble collapse attained does permit bubble collapse on the resistor and does permit refill to occur from more than one direction.
Three-sided barriers have been shown, but due to their configuration, have not resulted in improving resistor life or expulsion of static bubbles. See, for example, U.S. Pats. Nos. 4,502,060; 4,503,444; 4,542,389; and 4,550,326.